In some electronic circuits, isolation of one portion of a circuit from others is required. Transformers are widely used for isolation in AC circuits. Conventional cored pulse transformers have a primary winding and a secondary winding and work on the principle that energy can be efficiently transferred by magnetic induction from one winding to another winding by a varying magnetic field produced by the alternating current. Pulse transformers are used extensively for isolation, for example in MOSFET gate driver circuits, but have several serious shortcomings. Conventional cored pulse transformers are expensive, bulky, and can vary significantly from unit to unit in terms of electrical characteristics.
Core-less PCB transformers use primary and secondary windings on opposing sides of a PC board. Such transformers lack a magnetic core and have a relatively small number of windings, with the result that they have a relatively low magnetizing inductance and higher leakage inductance. The use of core-less PCB transformers can save expense, ensure greater uniformity among units, and avoid saturation problems. However, the use of core-less PCB transformers for isolation in circuits offers technical challenges as well. To avoid high primary side drive current associated with the low magnetizing inductance, these transformers are typically operated at switching frequencies within a range of about 7–11 MHz. Since most power transistors cannot be switched at such high frequencies, a PWM waveform cannot practically be sent directly across these core-less transformers. In order to address this problem, it is known in the arts to differentiate a PWM input signal waveform by subtracting it from a delayed version of itself. This produces a positive pulse indicative of the rising edge of the PWM input and a negative pulse indicative of the falling edge. These pulses are fed into the primary side of the transformer. A latch is used on the secondary side of the transformer. The positive pulse sets the latch and the negative pulse resets it, thus reconstructing the original PWM input signal. Although ideally this approach would substantially reproduce the input PWM signal at the output, it is very difficult to build a latch that is able to operate reliably in a noisy environment. Noise events can act to set or reset the latch. Such accidental operation of the latch can result in damage to the circuit. An additional problem is that inductive flyback from the pulse transformer can cause the latch to reset immediately after being set, or vice versa, also potentially causing damage to the circuit.
Due to these and other problems, improved pulse transformer driver circuits would be useful and advantageous in the arts.